The Gene: An Intimate History - Siddhartha Mukherjee Page 0,11

captured one of the essential truths about the nature of heredity. The transmission of heredity, as Aristotle perceived it, was essentially the transmission of information. Information was then used to build an organism from scratch: message became material. And when an organism matured, it generated male or female semen again—transforming material back to message. In fact, rather than Pythagoras’s triangle, there was a circle, or a cycle, at work: form begat information, and then information begat form. Centuries later, the biologist Max Delbrück would joke that Aristotle should have been given the Nobel Prize posthumously—for the discovery of DNA.

But if heredity was transmitted as information, then how was that information encoded? The word code comes from the Latin caudex, the wooden pith of a tree on which scribes carved their writing. What, then, was the caudex of heredity? What was being transcribed, and how? How was the material packaged and transported from one body to the next? Who encrypted the code, and who translated it, to create a child?

The most inventive solution to these questions was the simplest: it dispensed of code altogether. Sperm, this theory argued, already contained a minihuman—a tiny fetus, fully formed, shrunken and curled into a minuscule package and waiting to be progressively inflated into a baby. Variations of this theory appear in medieval myths and folklore. In the 1520s, the Swiss-German alchemist Paracelsus used the minihuman-in-sperm theory to suggest that human sperm, heated with horse dung and buried in mud for the forty weeks of normal conception, would eventually grow into a human, although with some monstrous characteristics. The conception of a normal child was merely the transfer of this minihuman—the homunculus—from the father’s sperm into the mother’s womb. In the womb, the minihuman was expanded to the size of the fetus. There was no code; there was only miniaturization.

The peculiar charm of this idea—called preformation—was that it was infinitely recursive. Since the homunculus had to mature and produce its own children, it had to have preformed mini-homunculi lodged inside it—tiny humans encased inside humans, like an infinite series of Russian dolls, a great chain of beings that stretched all the way backward from the present to the first man, to Adam, and forward into the future. For medieval Christians, the existence of such a chain of humans provided a most powerful and original understanding of original sin. Since all future humans were encased within all humans, each of us had to have been physically present inside Adam’s body—“floating . . . in our First Parent’s loins,” as one theologian described—during his crucial moment of sin. Sinfulness, therefore, was embedded within us thousands of years before we were born—from Adam’s loins directly to his line. All of us bore its taint—not because our distant ancestor had been tempted in that distant garden, but because each of us, lodged in Adam’s body, had actually tasted the fruit.

The second charm of preformation was that it dispensed of the problem of de-encryption. Even if early biologists could fathom encryption—the conversion of a human body into some sort of code (by osmosis, à la Pythagoras)—the reverse act, deciphering that code back into a human being, completely boggled the mind. How could something as complex as a human form emerge out of the union of sperm and egg? The homunculus dispensed of this conceptual problem. If a child came already preformed, then its formation was merely an act of expansion—a biological version of a blowup doll. No key or cipher was required for the deciphering. The genesis of a human being was just a matter of adding water.

The theory was so seductive—so artfully vivid—that even the invention of the microscope was unable to deal the expected fatal blow to the homunculus. In 1694, Nicolaas Hartsoeker, the Dutch physicist and microscopist, conjured a picture of such a minibeing, its enlarged head twisted in fetal position and curled into the head of a sperm. In 1699, another Dutch microscopist claimed to have found homuncular creatures floating abundantly in human sperm. As with any anthropomorphic fantasy—finding human faces on the moon, say—the theory was only magnified by the lenses of imagination: pictures of homunculi proliferated in the seventeenth century, with the sperm’s tail reconceived into a filament of human hair, or its cellular head visualized as a tiny human skull. By the end of the seventeenth century, preformation was considered the most logical and consistent explanation for human and animal heredity. Men came from small men, as large trees came from small cuttings.